I. Field of the Invention
The present invention relates generally to crankshafts and, more particularly, to a method and apparatus for constructing a multi-piece crankshaft.
II. Description of the Prior Art
In a conventional crankshaft, the crankshaft includes a segmented main shaft having an axis aligned with the axis of rotation of the crankshaft. One or more crankpins are also provided at a position radially spaced from, but parallel to, the main shaft. The piston is then secured to each crankpin which rotatably drives the crankshaft about its axis of rotation.
Since the crankpins are radially offset from the axis of rotation of the crankshaft and attached to a piston, counterweights are conventionally provided for securing the main shaft to the crankpin so that the overall weight of the crankshaft has a balanced rotation. In many previously known crankshafts, the entire crankshaft is made from a single heavy body of cast metal which is then machined so that the main shaft, crankpins and counterweights are of a one-piece construction. Machining such crankshafts, however, is necessarily expensive which increases the overall cost of the crankshaft.
There have, however, been crankshafts which are constructed from multiple pieces for low cost construction. Such crankshafts are oftentimes used in small two cycle engines although they can also be used in other types of engines or compressors.
Typically, in these multi-piece crankshafts, the counterweights are constructed from an inexpensive material, such as powdered metal or steel stampings, and the counterweight has holes formed through it corresponding to the position of the crankpin and main shaft. The crankpin and main shaft are then constructed from conventional round stock. The ends of the main shaft and crankpin are knurled, splined or otherwise deformed and pressed into the openings formed in the counterweight to thereby form the crankshaft.
One primary disadvantage of these previously known multi-piece crankshafts is that the main shaft and crankpins must be parallel to each other within very high tolerances. However, during the pressing operation, the main shaft and crankpins often become skewed relative to each other and the resulting crankpin assembly must be either corrected by bending the crankpin and main shaft relative to each other or, in some cases, discarded as scrap. Furthermore, these prior art crankshafts are also subject to failure from push out and twisting of the shaft relative to the counterweight.
The present invention provides a multi-piece crankshaft assembly and method for making the same which overcomes all of the above-mentioned disadvantages of the previously known multi-piece crankshaft constructions.
In brief, the crankshaft assembly of the present invention comprises a crankpin and main shaft, both of which are cylindrical in cross-sectional shape. One end of the main shaft includes an indentation, such as a spline, knurl or the like. Optionally, one or both ends of the crankpin also include an indentation.
A multi-piece crankshaft of the present invention further includes a generally planar reinforcing plate constructed of a high strength material, such as steel. This plate, furthermore, includes a first and second throughbore which are spaced from each other.
One end of the crankshaft is press fit into one of the openings of the plate so that the plate lies in a, plane generally perpendicular to the axis of the crankpin. The cross-sectional size and shape of the other plate opening, however, is larger than the end of the main shaft. Thus, when the end of the main shaft is positioned within the second plate opening, a clearance exists between the second plate opening and the end of the main shaft.
The crankpin and main shaft are then positioned in cylindrical positioning recesses in a mold having a mold cavity corresponding to the desired shape of the counterweight. Furthermore, with the crankpin positioned within its positioning recess, the plate is disposed within the mold cavity. Similarly, with the main shaft positioned within its positioning recess in the mold, one end of the main shaft is positioned through the second opening in the plate.
In practice, the positioning recesses of the mold are machined with high precision which ensures parallelism between the main shaft and crankpin once the main shaft and crankpin are positioned within their positioning recesses. Likewise, the radial spacing between the crankpin and main shaft is maintained with a high degree of precision.
With the crankpin, main shaft and plate positioned within the mold as described above, the mold cavity corresponding to the counterweight is then filled by pouring, injection or the like with a liquid heat setting material, such as zinc. This liquid material not only fills the mold cavity, but also the indentations in either or both of the crankpin and the main shaft as well as through openings preferably formed in the plate. Consequently, upon hardening of the material, the material fills the indentations in one or both of the crankpin and main shaft thus precluding either rotation or longitudinal movement between the counterweight and both the crankpin and main shaft.
Since the main shaft and crankpin axe maintained parallel to each other with a high degree of precision, once the liquid material has set, parallelism between the crankpin and main shaft is established for the completed crankshaft assembly. Furthermore, since this liquid material also encases the plate, the plate increases the overall structural strength of the counterweight over that obtainable from the heat setting material alone.
In a further embodiment of the invention, one end of the crankpin is press fit into one end of the plate while the other end of the plate terminates short of the main shaft.